1. Field of the Invention
The present invention relates to a hydraulic control system for controlling the applied states of frictional engagement units in an automatic transmission and, more particularly, to a system for controlling the supply of lubricating oil to the frictional faces of the frictional engagement units.
2. Related Art
There has been widely known an automatic transmission which is constructed to change gear ratios by applying/releasing frictional engagement units such as a friction clutch or a brake suitably to change the transmission path of a torque. In the automatic transmission of this kind, any of the frictional engagement units is applied or released to change the gear ratios. During the application/release, the applying force is gradually changed by the oil pressure to change the transmission torque capacity continuously. As a result, the output torque is smoothly changed, thereby preventing any physical shock, as might otherwise be felt by the driver of a vehicle when the gear ratios are changed.
These actions of the frictional engagement unit at the time of changing the gear ratios are caused by the slippage which occurs transiently in the frictional engagement unit to absorb the inertia force. By utilizing this function positively, the vehicle can be started without any shock. In the prior art, more specifically, a torque converter is arranged upstream of a gear speed change mechanism. Even if the torque converter is eliminated and replaced by a friction clutch, however, the output torque can be smoothly raised by controlling the friction clutch in a slipping manner at the start, thereby allowing the vehicle to start without any shock.
By controlling the frictional engagement unit in the slipping manner, the shock, as might otherwise be caused at the start, can be avoided. On the other hand, a heat is generated on the frictional face of the frictional engagement unit in which the slippage is occurring. In other words, the kinetic energy is absorbed as the thermal energy, which requires to take any steps to avoid the burnout. An example of these steps is disclosed in Japanese Patent Laid-Open No. 11-125273 (JP-A11-125273). The system as disclosed is an automatic transmission having a planetary gear mechanism for switching the forward and backward runs and a starting clutch which are arranged upstream of a continuously variable transmission. These planetary gear mechanism and starting clutch are accommodated in a housing filled with lubricating oil. As an engine is started to drive an oil pump, the lubricating oil is circulated in the housing and supplied to the starting clutch continuously to carry away the frictional heat, thereby preventing the temperature rise of the starting clutch and the deterioration in its durability.
In the system disclosed in the above-specified Japanese Patent Laid-Open, the starting clutch is controlled in the slipping state at the start of the vehicle and then kept in a completely applied state as accompanied by no slippage. While the engine is active, on the other hand, a substantially equal quantity of the lubricating oil is constantly circulated in the housing, considering the slipping state at the start, and supplied to the starting clutch.
Thus, the starting clutch can be sufficiently cooled at the start. However, even when the starting clutch is completely applied to establish no slippage, the supply of the lubricating oil is continued as before. This action makes the oil pump drive excessively for circulating the lubricating oil continuously, thereby permitting the motive power loss to occur. Moreover, the lubricating oil is excessively stirred to raise the temperature, thereby causing a disadvantage that the deterioration of the lubricating oil advances.
A main object of the invention is to provide a hydraulic control system for an automatic transmission which can cool frictional engagement units to be controlled in a slipping state and in an applied state, as accompanied by no slippage, without causing any motive power loss.
Therefore, the hydraulic control unit of the invention is provided with a mechanism for increasing the amount of lubricating oil to be supplied to the frictional face of a frictional engagement unit, by utilizing drain oil which is established while the frictional engagement unit is kept in the slipping state.
The mechanism for increasing the supply of the lubricating oil to the frictional face includes a mechanism for supplying the drain oil as it is to the frictional face or a change-over valve on which the drain oil acts as a signal pressure to increase the amount of the lubrication oil to be supplied.
In the invention, for example, a predetermined frictional engagement unit is controlled into the slipping state. Simultaneously with this control, another control is executed to establish the drain oil. This drain oil is supplied to the frictional face of the predetermined frictional engagement unit, as controlled into the slipping state, to increase the amount of oil supplied to the frictional face to promote the cooling. When the control into the slipping state is not executed, on the other hand, the drain oil is not supplied to the frictional face to decrease the amount of circulated oil, thereby reducing the consumption of the motive power.
In the invention, on the other hand, simultaneously with the control to bring the predetermined frictional engagement unit into the slipping state, another control is executed to establish the drain pressure. According to the control to bring the predetermined frictional engagement unit into the slipping state, the drain oil is supplied as the signal pressure to a change-over valve. This change-over valve opens a lubricating oil passage having a larger sectional area for supplying an increased amount of the lubricating oil to the frictional face to be controlled into the slipping state, thereby promoting the cooling of the frictional face. When the slip control is not executed, on the other hand, the lubricating oil passage having a larger sectional area is closed to decrease the supply of the lubricating oil, thereby reducing the consumption of the motive power.
The invention is further provided with an oil passage construction in which the oil pressure for controlling the frictional engagement unit into the slipping state is shut off when the drain oil is not utilized for increasing the amount of lubricating oil.
In the invention, therefore, when the drain oil cannot be circulated, that is, a sufficient lubricating oil cannot be supplied to the frictional face of the frictional engagement unit to be controlled into the slipping state, the oil pressure for controlling the frictional engagement unit into the slipping state is shut off. In short, the control of the predetermined frictional engagement unit into the slipping state is suppressed. Even if the lubrication for cooling the frictional face is troubled, therefore, the slipping control itself of the frictional engagement unit is suppressed to prevent the trouble such as the burnout of the frictional engagement unit, as might otherwise be caused by the frictional heating, in advance.
In the invention, moreover, the predetermined frictional engagement unit is controlled into the slipping state to transmit the torque while the vehicle is running or into the applied state, as accompanied by no slippage, to transmit the torque while the vehicle is not running.
In the invention, therefore, as the load on the frictional engagement unit is raised in the slipping control, the supply of oil is increased accordingly to avoid the burnout of the frictional face. In the case of the control into the applied state as accompanied by no slippage, on the other hand, the load on the frictional engagement unit is lowered to reduce the amount of supplied oil, thereby effectively avoiding the excessive circulation of the oil and the accompanying loss of the motive power.
In the invention, moreover, it is possible to have in the same direction the start which is effected by connecting an internal combustion engine and an electric motor as a motive power source and by controlling the frictional engagement unit into the slipping state while inputting the motive power from the internal combustion engine and the start which is effected by inputting the motive power from the electric motor while releasing the frictional engagement unit.
In the invention, therefore, when the vehicle is started by the motive power of the internal combustion engine while being accompanied by the slippage of the predetermined frictional engagement unit, the amount of oil to be supplied to the frictional engagement unit is increased to avoid the temperature rise, as might otherwise accompany the slippage with the high loading motive power, to prevent the trouble such as the burnout in advance. When the vehicle is driven by the electric motor without being accompanied by the slippage of the frictional engagement unit, on the other hand, the amount of oil to be supplied to the frictional engagement unit is decreased so that the unnecessary consumption of the motive power, as might otherwise be caused by supplying excessive oil, can be prevented.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are presented for the purpose of illustrations only and are not intended as a definition of the limits of the invention.